1. Field of the Invention
The present invention relates, in general, to a semiactive suspension fixed to wheels for absorbing and reducing uncomfortable effects such as vibrations in a vehicle and, more particularly, to a system and method of controlling such a suspension suitable for operating processing output signals from acceleration sensors while a vehicle is in a straight travelling or cornering mode, thus checking road surface conditions and vibrations of the vehicle and independently controlling the damping forces of dampers for all four wheels.
2. Description of the Prior Art
In order to provide comfortable and smooth ride, vibration caused by road surfaces have to be effectively absorbed and reduced. In the event of vertical vibrations, passengers easily feels vibrations at a frequency band of about 5-7 Hz, which is the resonance frequency band of the human autonomic nervous system. Meanwhile, the frequency band of horizontal vibrations, sensed by a person, is lower than that of the vertical vibrations.
With this regard, it is necessary for a vehicle to reduce uncomfortable effects of going over uneven road surfaces, unpaved road surfaces, concrete-paved road surfaces or junctions of bridge spans. In order to achieve the above object, semiactive suspensions, individually having a variable spring constant or variable damping force, are widely used in various vehicles.
FIG. 1 is a block diagram showing the construction of a typical automatic control system for semiactive suspensions. As shown in the drawing, the control system has a plurality of signal input members: a vertical acceleration sensor 10, a speed sensor 20, a throttle position sensor (TPS) 30, a steering angle sensor 40 and a brake switch 50. The vertical acceleration sensor 10 checks the vertical acceleration of a vehicle. The speed sensor 20 checks the speed of the vehicle. The throttle position sensor 30 checks the opening angle of the throttle valve of an engine. The steering angle sensor 40 checks the steering angular velocity of a steering system. The brake switch 50 checks the operation of a brake system. In the operation of the above control system, the signals from the above signal input members 10, 20, 30, 40 and 50 are processed by an electronic control unit (ECU) 100 in accordance with a preset program. After processing the signals, the ECU 100 determines the desirable damping forces for controlling the variable dampers 200, which are fixed to the wheels of a vehicle. The above semiactive suspension thus provides comfortable ride to a driver and passengers and secures steering safety of the vehicle.
The above automatic control system controls three modes of the variable variable dampers 200: soft, medium and hard modes. In order to control the damping mode, the control signals from the ECU 100 are applied to four damper actuators 210, 220, 230 and 240. Upon receiving the control signals from the ECU 100, the actuators 210 to 240 rotate the control rods of associated shock absorbers thus changing the cross-sectioned areas of the oil passages for the shock absorbers to convert the damping mode.
In FIG. 1, the reference number 60 denotes a mode select switch.
The ECU 100 of the control system comprises a plurality of independent control logics, which independently control the semiactive suspension between the three damping modes in accordance with conditions of a vehicle as follows.
First, an anti-bounce control logic of the ECU 100 checks bumps or rough road surfaces and selectively converts the damping mode of the dampers 200 into the medium mode thus preventing bouncing of the vehicle when both the speed of a vehicle is-not less than V.sub.1 Kph and the vertical acceleration at the vehicle's center of gravity (console box) is not less than G.sub.1 g. After the lapse of t.sub.1 seconds from the above conditions, the anti-bounce control logic returns the medium damping mode into the standard damping mode.
Second, an anti-shake control logic of the ECU 100 converts the damping mode of the dampers 200 into the hard mode when the speed of the vehicle is not higher than V.sub.2 Kph, thus making the vehicle barely shake while a passenger gets in or out of the vehicle or when the vehicle is being loaded or unloaded. When the vehicle maintains a speed of not less than V.sub.21 Kph for at least t.sub.2, the anti-shake control logic returns the hard mode into the original mode.
The anti-shake control logic converts the damping mode into the medium mode when the vehicle, travelling at a high speed, maintains a speed of not less than V.sub.3 Kph for at least t.sub.3, thus securing steering safety in the above conditions. When the speed of the vehicle is not higher than V.sub.31 Kph, the anti-shake control logic returns the damping mode into the original mode.
Third, an anti-squat control logic of the ECU 100 reduces the vertical vibrations of the front end of the vehicle when the vehicle starts at a low speed. In order to achieve the above object, the anti-squat control logic converts the damping mode into the medium mode when both the speed of the vehicle is less than V.sub.4 Kph and the opening angle of the throttle valve is higher than .theta..sub.4 Deg. Meanwhile, the anti-squat control logic returns the damping mode into the original mode in the event of either the lapse of t.sub.4 seconds or when the speed of the vehicle is higher than V.sub.41 Kph.
Fourth, an anti-dive control logic of the ECU 100 reduces the diving of the front end of the vehicle when the vehicle, travelling at a mid to high speed, suddenly stops. In order to achieve the above object, the anti-dive control logic converts the damping mode into the hard mode, when the vehicle is braked at a speed of not less than V.sub.5 Kph and a stop lamp switch is turned on. Meanwhile, the anti-dive control logic returns the damping mode into the original mode in the event of the lapse of t.sub.5 seconds after the above conditions.
Fifth, an anti-roll control logic of the ECU 100 is for securing steering safety of the vehicle. In order to achieve the above object, the anti-roll control logic converts the damping mode into the hard mode when the steering angle is higher than a reference angle with a speed of the vehicle being not less than V.sub.6 Kph. Meanwhile, the anti-roll control logic returns the damping mode into the original mode in the event of the lapse of t.sub.6 seconds after the above conditions.
However, the above automatic control system for semiactive suspensions is problematic in that it provides a low damping force for controlling the suspensions of a vehicle, thus failing to either precisely controlling the suspensions or effectively reducing uncomfortable ride. Another problem experienced with a conventional control system is that it has a slow reaction thus failing to rapidly react to the abrupt change of the conditions of a vehicle. The typical automatic control system for semiactive suspensions fails to provide provide neither a better comfortable ride nor reliably secures steering safety of a vehicle in comparison with a typical manual control system for semiactive suspensions.